mames minimum specs to run anything thats playable are
p4 with ht 2.8 or better
512 ram (1 gig recccomended)
at least a 256 gfx card
i have built a ton of mame rigs using old compaq sff workstation pc's with that setup and only notice slowdown on the games that are marked as having problems anyway
in reality for future mame updates i would go above the minium specs
for my higher end machines i use any dual core amd cpu (intel is no better than amd for a mame rig and amd is usually half the price)
i usually get these
http://www.microcenter.com/single_product_results.phtml?product_id=0355993
and usually get one of these 2 motherboards depending on whats on sale at the time(microcenter also is doing a free mobo with cpu purcase right now)
http://www.microcenter.com/single_product_results.phtml?product_id=0354008
http://www.microcenter.com/single_product_results.phtml?product_id=0350745
as for ram its pretty cheap if you go with the above mobo's
you dont need anything fancy
and only need 1 stick if you get these
http://www.microcenter.com/single_product_results.phtml?product_id=0331256
as for graffix cards i have been using these as they are cheap (19.00 after rebate)but have 1gb memory
http://www.microcenter.com/single_product_results.phtml?product_id=0354355
as for harddrives
if you are just doing mame i recomend getting a ss drive there is a nice 64 gb one that will give you enough room for your os and a complete working game rom set
http://www.microcenter.com/single_product_results.phtml?product_id=0351760
if you must use other emus as well it really just depends on how much youo want to spend and how big you need it to be
dont buy any hd's that are under 7500 rpm
as for an os use win xp stay away from the lite versions and hacked versions ans there are usually striped drivers and system componentss on the lite versions and the hacked versions usually have a crap ton of not need apps included wich make them bloated and slower
for a front end use
game ex hyper spin is a pia and it sucks trying to use joys and buttons to nagavite thrue the damn wheel
game ex also has a sweet juke box feature as well and setup can be don by my 3 year old lol
http://www.gameex.net/
also for a seamless boot into your frontend with no signs of windows use this app
http://www.gameex.net/download/instantsheller.zip
point it to the exe of your front end and if you need a pasword to login click the setaouto login button and bam next time you reboot you will see nothing of windows
if you want to change the art for the boot screen just create a picture 640x480 and name it the same as the boot immage in the zip package
here is the info page for the shelling app\
http://www.gameex.net/Community/InstantShellerandshellingguide.aspx